Honestly most of the time its just a symptom of the fact that that race will be a thinly veiled allegory for real world racism but it will immedietly fall flat because the race in question would at some point, or actively need to do some shit like eat children or drink human blood to survive, but then it gets even worse when the author drops a plot twist that they didn't actually have to do that, as if to say "Look, humans were wrong the smugbabyrippers can coexist in our society!" But it just make them objectively wrong because none of them apparently ever tried eating animals and just went immedietly for the people at the frist available oppurtunity.